A number of intricate and complex signal processing functions, such as the generation of Fourier transforms, are now being effected by surface acoustic wave (SAW) systems. In one example of a SAW system a transmitting means (typically one or an array of small input transducers) excites surface waves on a wave propagating substrate. The waves interact to provide spatially or temporally modified wave variations that are reproduced in receiving transducers. For example, an array of input transducers may be configured so that, in response to a wideband signal input, they generate diffraction patterns forming beams at one or more different angles dependent upon the frequencies present in the input band. Output transducers at specific locations in the beam paths can then detect the signals, so that the system functions as a unique form of spectrum analyzer. Using modern photolithographic techniques and certain available materials, small but wideband devices can be constructed that are readily reproducible and have the further advantage of preserving phase coherence.
Sidelobes, scattering, and other spurious transmissions inevitably are present in transmitted waves to some degree. Because of the spatial operative characteristics of SAW devices, they may distribute such unwanted wave energy in known locations on the substrate, at different points in time. Where the conditions are invariant, the waves may be dissipated by an attenuating or absorbing layer, such as a rubbery compound. This technique is not applicable, however, when conditions vary, as is almost inherently the case with a wideband system. For example, it may be desirable to track a signal of varying frequency, while excising signals of other frequencies, so as to provide dynamic filtering. Whether used for such elemental functions as signal modulation, or for system purposes such as improvement of characteristics, such capabilities would offer new dimensions for signal control and processing functions.
It is well known in advanced signal processing techniques to use surface acoustic waves to modulate light in transforming signals. A related but different technique is to utilize a special configuration of SAW substrate, in conjunction with photoconductive layers, to convert an optical image to electrical signals. Such systems are discussed in articles entitled "Two-dimensional Photoacoustic Image Processing With Longitudinal Waves" by S. T. Kowel et al, in the 1978 Ultrasonics Symposium Proceedings, pp. 258-262 and "DEFT Sensor Operating At 100 MHz" by S. T. Kowel et al, in the 1979 Ultrasonics Symposium Proceedings, pp. 85-89. These articles describe how surface acoustic waves enable scanning of a semiconductor light sensor positioned on the SAW substrate surface, and the manner in which Fourier spectra may be obtained. A system in which optical energy is utilized to modify the characteristics of a SAW device itself is described in issued U.S. Pat. No. 3,911,381, to Robert E. Brooks et al, entitled "Tunable Acoustic Wave Propagation Device", Oct. 7, 1975. In this sytem, a photoconductive overlay on a SAW substrate is used in a system in which a coupler is disposed between transmitting and receiving elements. In this arrangement, the properties of the multi-strip coupler are modified by a static pattern presented by a photomask, a hologram, or the interference pattern between a pair of adjustable impinging coherent wavefronts. Thus the strips in the coupler are not defined by deposited conductive metallic or other elements, but by the higher conductivity regions of the photoconductive layer, under the illuminating light pattern. While this system is advantageous for the intended use, it does not otherwise serve to extend the capability of surface acoustic wave systems.